1. Field of the Invention
The present invention relates to methods for preparing alkali cellulose and water-soluble cellulose ether having the low insoluble fiber content.
2. Description of the Related Art
Water-soluble cellulose ether is produced by reacting cellulose having, in the molecule thereof, both a crystalline portion and a non-crystalline portion with an etherifying agent to convert the crystalline portion into the non-crystalline portion and thereby making the cellulose ether soluble in water. It is said that crystallinity of cellulose owes to a hydrogen bond between intramolecular hydroxyl groups attributable to the skeleton structure of the cellulose molecule. This hydrogen bond, which is firm, disturbs hydration with a water molecule in water and becomes a cause for making the cellulose water-insoluble. Cellulose ether is prepared by converting the cellulose into alkali cellulose with an aqueous solution of an alkali such as NaOH, thereby breaking its crystallinity; and reacting the alkali cellulose with an etherifying agent to substitute the hydroxyl group of the cellulose by the etherifying agent. The resulting alkali cellulose however does not completely lose crystallinity. It is industrially difficult to substitute all the hydroxyl groups of the cellulose by raising the degree of substitution for ether so that commercially available cellulose ethers are water-soluble but have a water-insoluble portion. The water-insoluble portion sometimes has a fiber scale of pulp, which is a raw material cellulose, exceeding even 1000 μm.
Water-soluble cellulose ether becomes tacky when dissolved in water so that it is used for a thickener for transparent shampoos and rinses, hair styling agents, eye drops, detergents for contact lens and the like. For example, methyl cellulose or hydroxypropyl cellulose, which is water-soluble cellulose ether, has a hydrophilic group and a hydrophobic group in the molecule thereof and thereby shows interfacial activity. It is therefore used as a suspension stabilizer in suspension polymerization of vinyl chloride or vinylidene chloride and becomes useful also as a raw material for transparent plastic wrap for domestic use. Products in such applications are desirably transparent. Unless water-soluble cellulose ether is water-soluble and transparent at the molecular level with regard to an aqueous solution of the water-soluble cellulose ether, defective portions appear in the products and they may lead to inferior transparency or inferior function. An aqueous solution of cellulose ether desirably has a high viscosity. Cellulose ether having a high viscosity has higher insoluble fiber content than cellulose ether having a low viscosity so that it is thought to be difficult to obtain a transparent product.
With a view to overcoming the above-described problems, Japanese Patent Application Examined Publication No. 53-12954/1978 proposes a method comprising a step of allowing a raw material pulp to adsorb an aqueous alkali solution having a concentration of 15 to 75% by weight at 5 to 80° C., and then pressing the resulting pulp within 10 seconds to remove an extra portion of the aqueous alkali solution, a step of repeating the above step to obtain the corresponding alkali cellulose, and a step of reacting the alkali cellulose with an etherifying agent.
Japanese Patent Application Unexamined Publication No. 10-259201/1998 proposes a method comprising steps of impregnating a pulp having dichloromethane extract content of 0.07% by weight or less with sodium hydroxide, pressing the resulting pulp to obtain the corresponding alkali cellulose, and then etherifying the alkali cellulose.
According to Japanese Patent Application Unexamined Publication No. 2001-354701, cellulose ether is produced by a method comprising steps of pulverizing a pulp sheet having a sheet density of 0.4 to 1.0 g/ml into powders having an average particle size of 1000 μm or less, adding an alkali to the powders to yield the corresponding alkali cellulose, and then reacting the alkali cellulose with methyl chloride, propylene oxide and the like.
According to A. W. Anderson and R. W. Swinehart, Tappi, Vol. 39, No. 8, 548-553, August, 1956, presented is a method for producing alkali cellulose, comprising a step of impregnating a pulp sheet having a sheet density of 0.47 to 1.17 g/ml in a bath containing an alkali solution for 0.5 to 4.5 seconds.